1. Field of the Invention
The present invention relates to a goggle type display apparatus, and more specifically, to a goggle type display apparatus mounted on the user's head to realize virtual reality.
2. Description of the Related Art
In recent years, increasingly rapid progress has been made in realizing less expensive computers with higher performance. With this progress, many kinds of technology which had been long thought impossible are now being realized. One type of such technology which has especially attracted attention and of which progress has been greatly expected is virtual reality (VR) technology.
The VR technology is an integrated form of many kinds of technology including display technology, computer technology, sensor technology, and acoustic system technology. Among them, the improvement in performance of computers has greatly contributed to the realization of the VR technology. Thus, the computer technology occupies a larger portion of the VR technology, even though many other novel arts of technology are involved in the VR technology.
According to VR technology, by being exposed to images, sounds, and the like produced by electronic means such as a computer, the viewer feels the images which do not exist in reality as if they actually exist. Thus, the viewer can enter the image space created by the computer and enjoy various experiences.
According to VR technology, the senses of the viewer themselves work as interfaces between the computer and the viewer, so that the viewer can experience events in the virtual space created by a computer in a purely natural manner. In this meaning, the VR technology also idealistically works as a man-computer interface.
Thus, VR technology provides an opportunity of experiencing easily and naturally what otherwise could not have been experienced. For this reason, the application of VR technology in the future to the fields of amusement, education, computer interface, and the like is greatly expected. The general outline of VR technology is described, for example, in M. Hirose, "Trend of Virtual Reality Technology", Kogaku, vol. 21, No. 9, pp. 597-604 (1992).
The visual sense is the most important sense for human beings. It is said that approximately 80-90% of information obtained by recognizing the external world is recognized by the visual sense. For this reason, in order to realize high level virtual reality, a high level technique of forming visual information is indispensable. In other words, in order to obtain virtual reality with more naturalness using the VR technology, it is indispensable to use a display system superior in VR display.
As for the display technology for VR, several display systems have been conventionally proposed. Among these systems, a head mounted display (HMD) system in which a goggle type display apparatus or the like is mounted on the head of the viewer has attracted wide attention as a significantly promising system. The following display methods are known for the goggle type display apparatus used for the HMD system.
1) CRT mirror reflection method
Because the display apparatus adopting this method provides images with high precision, this display method has been studied as an epoch-making technique in which the pilot in the cockpit can handle a large amount of information in an integral manner. This method is described in detail in J. Sellers, "Helmet-Mounted Display Electronics for Evaluating Virtual Panoramic Display Systems", SID 91 Digest, pp. 491-494 (1991). FIG. 15 shows the concept of a virtual panoramic display (VPD) system, a typical application of this method.
The VPD system has been proposed as a system for providing various kinds of information to the pilot in the cockpit. A helmet 101 which is a display apparatus of the VPD system includes two CRTs 102, optical systems, and an HPS or a sensor for detecting the position and orientation of the user's head. A unit for controlling the display, and the like are disposed in a cockpit 100. The VPD system also includes a cooling unit for the helmet 101, an image generating computer, a control computer, and the like. With these components, the size of the VPD system is very large. Since the CRTs 102 are mounted in the helmet 101, the entire VPD system inevitably becomes large and heavy. The cooling unit for cooling heat generated from the CRTs 102 is additionally required. For these reasons, the application of the VPD system has been limited only to a specific industrial field and to the military field.
2) Transmission type LCD method
Since the transmission type LCD is thin and light, a small and light HMD system can be realized by adopting this method. A display apparatus adopting this method is commercially available from Visual Programming Language (VPL) Corp. under the product name of "Eyephone". This display method is described in detail in J. Nomura, Television Society Paper, vol. 46, No. 6, pp. 689-693 (1992) and T. Uchida and T. Miyashita, "A stereoscopic Display Using LCDs", SID 86 Digest, pp. 440-443 (1986). FIG. 16 shows the principle of this display method. Though FIG. 16 only shows one set of a major portion of an apparatus for one eye, the actual apparatus should include two sets for both the left and right eyes.
An LCD 120 for the left or right eye is illuminated by an illumination optical system 122 composed of a light guiding plate 122a and a fluorescent lamp 122b. Information visualized by the LCD 120 is then received by the viewer through an eyepiece optical system 121. The respective images reflected on the left and right eyes have been generated by a computer as images to be viewed with the both eyes simultaneously. By this simultaneous viewing, the parallex between the left and right eyes is utilized to realize a stereoscopic display. The image is generated by the computer according to the information including the position and orientation of the viewer's head and the position of a viewer's hand detected by a magnetic sensor, an optical fiber sensor, and the like.
This display method has the following problems. Since this method uses a transmission type LCD where an increase in the number of pixels is limited, images with high precision can not be obtained, and thus the reality of the images is not satisfactory. Actually, the number of pixels of Eyephone is only 360.times.240 pieces. With such a small number of pixels, when a large number of letters are simultaneously displayed on one screen, each letter cannot be displayed clearly. Further, since the LCD used in this display method is of a direct-view type, there arise problems relating to the characteristics of the visual sense, such as the dependency of the display on the angle of visibility and the uniformity of display. For example, in order for the viewer to satisfactorily enjoy the visual realism according to the characteristics of the visual sense, it is required to maintain the display under good conditions over the range defined by the angles of visibility of 50.degree. toward left and right, 35.degree. toward upward, and 50.degree. toward downward. However, such a broad range of the angles of visibility cannot be realized by the direct-view type LCD.
3) LED (light emitting diode) scanning method
FIG. 17 shows the principle of display for a display apparatus adopting the LED scanning method. In this display apparatus, a light pattern is emitted from a one-dimensional LED array 130. This pattern is oscillated by an oscillation mirror 131, so that a two-dimensional pattern is obtained by the afterimage effect of the eyes. More specifically, a one-dimensional pattern (in a y direction) formed by the on/off of LEDs of the LED array 130 is magnified by a magnifying mirror 132 and scanned in an x direction by the oscillation mirror 131, thereby converting the one-dimensional pattern into a two-dimensional image.
Though the display apparatus adopting this method is small and light, it has problems as follows. Since only a binary display is allowed, an intermediate tone is not available. Further, since the precision of the display apparatus depends upon the density of the LED array 130, or the number of LEDs in the array, high precision display is not possible. A display apparatus adopting this method is commercially available under the product name of "Private Eye".
Hereinbefore, the typical goggle type display apparatuses used for the HMD system have been described. These conventional apparatuses have the following problems.
In order to obtain the visual realism by the HMD system, the following four requirements should be satisfied: 1) a broad field of vision, 2) natural stereoscopic display, 3) high precision display, and 4) a small and light apparatus.
The CRT mirror reflection method satisfies requirements 1) through 3), but does not satisfy requirement 4) because it is difficult to make the CRT itself thin and light.
The transmission type LCD method can be made small and light. However, a broad range of the angles of visibility is not obtained. Moreover, the improvement in precision of the display is limited. This is because, as the size of pixels of the transparent type LCD is reduced, the aperture of the LCD lowers, and thus the luminance of the display is lowered. Accordingly, the size of the pixels cannot be reduced beyond a predetermined level.
The trouble of the limited improvement in precision of the display for the transmission type LCD will be described in more detail, taking an LCD using conventional TFTs as an example, as follows.
Conventionally, polysilicon (p-Si) and amorphous silicon (a-Si) are used for TFTs. An LCD using such TFTs has the following problems.
The mobility of charges of an a-Si TFT is generally 0.1-0.5 cm.sup.2.V.sup.-1.s.sup.-1. This means that the ON resistance of the transistor is high, and, as a result, the size of the TFT should be large. The mobility of a p-Si TFT is higher than that for the a-Si TFT, but it is still as low as approximately 50-100 cm.sup.2.V.sup.-1.s.sup.-1. Therefore, the size of the p-Si TFT should also be large. The p-Si TFT has another problem of having a small OFF resistance. This is another factor for requiring large size TFTs.
For the a-Si TFT and p-Si TFT, when the polarity of the voltage applied through the source and drain thereof is alternately inverted, the electrical properties of the TFT differ between the different polarities. This produces a non-uniform display on the LCD screen, causing flickering and twinkling on the screen. In order to solve this problem, a driving method in which the polarity of the applied voltage is switched every scanning line (1H line inversion driving) is generally adopted. According to this method, however, since the polarity of the voltage applied to the pixels is different for every scanning line, a large electric field may be formed between adjacent pixels, resulting in disordering the orientation of the liquid crystal. Actually, when .+-.5 V is applied as the driving voltage, the distance between the pixels should be 10 microns or more. With this distance, the displayed image is prevented from being influenced by the electric field generated between adjacent pixels.
As described above, in the conventional LCDs using TFTs, the size of the TFTs cannot be reduced so much. Therefore, when the size of each pixel is made smaller so as to obtain high precision display, the TFT occupies a larger area of the pixel and thus the aperture of the LSD lowers. When the pitch of the pixels is 20 microns or less, the aperture is substantially zero. For this reason, high precision display is not possible for the goggle type display apparatus adopting the transmission type LCD method, and thus display of natural images cannot be obtained.
In addition, in a conventional color LCD, color filters are disposed on the LCD, and one color among red (R), green (G), and blue (B) colors is allocated to each pixel. Since a color of one point is produced by synthesizing three pixels of R, G, and B colors, the displayed image is coarser.
The LED scanning method can realize a broad range of the angles of visibility, and can be small and light. However, since only binary display is allowed, an intermediate shade of color is not available, and thus the display of natural images is not possible. Moreover, since the precision of the display depends upon the density of the LED array and the increase in the density is limited, high precision of the display is not possible. For this reason, also, the display of natural images cannot be obtained.